I still don't quite understand what made it so special and noteworthy outside of it having Kojima's name on it and it being tangentially related to Silent Hill.
There was nothing like it at the time.
* Ultra-realistic graphics/sound in a horror game (teaser).
* Extremely intimately close and highly "grounded" setting. Just a hallway and a bathroom.
* Unnerving & untrustworthy surroundings.
With regards to the zeitgeist at the time:
* Major secretive and super obscure treasure hunt. Sony just drops this high-quality teaser on PSN with a small mention in Gamescom (?), from an unknown dev team.
* After days (hours?) of a major community effort to "beat" the game people finally get the ending and blows everyone away when the secret is revealed: new Silent Hill game, Kojima Productions, Guillermo Del Toro, Junji Ito, Norman Reedus. 🤯
The only big downside is actually getting the ending is an exercise in frustration.